A Brief History of the Tilt-rotor Aircraft

"Rock Bottom" (rockbottom81) 10/01/2013 at 19:37 • Filed to: Planelopnik, Tunnelopnik, Geeklopnik

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I think it’s time I profess my love for tilt-rotor aircraft. Enjoy/suffer through this!

In The Beginning

As helicopters progressed past their WWII infancy, researchers started to run into the expected speed limitations inherent to all rotorcraft that generate 100% of their lift and thrust from a rotor in edgewise flight. In addition to the push for more speed, there was a push for more range.

There were several ways to get improvements in speed and range and they all have their merits. Sikorsky, for example, bet on coaxial helicopter technology (see Sikorsky S-69 ABC and X-2). Lockheed spent a bunch of time on compound helicopters (like the XH-51 and AH-56 Cheyenne). Piasecki/Vertol/Boeing got good at tandem rotor machines (like the CH-46 and CH-47). Bell tried their hand at tilt-rotor machines.

It all started with the Bell Model 200. Well, actually it started in the 1930s with the British patent for the Baynes Helicopter (never built), which preceded the Transcendental Model 1-G. The 1-G was the first flying tilt-rotor and it completed about 100 test flights before it was crashed into Chesapeake Bay. Bummer.

Bell XV-3

Enter Bell Aircraft. In 1951, before the 1-G got into the air, the US Army and the US Air Force released an RFP (Request For Proposal) which led Bell to dump a bunch of engineering horsepower (and cash) on this wacky “convertiplane” concept. Bell designed and built a machine called the Model 200, which was given the “X Plane” designation of XV-3. The purpose of the XV-3 was to prove that a convertiplane (later called a tilt-rotor) could be practical and controlled. It wasn’t necessarily meant to push the limits… yet…

The XV-3 first flew (in helicopter mode) in 1955. It vibrated badly due to a little thing called rotor dynamic instability. Several ground tests and a series of wind tunnel tests (where the whole aircraft was mounted and flown in the NASA Ames 40x80 wind tunnel) were performed and several proprotor (the whirly thing at the end of the wing) configurations were used. Once the engineers understood the causes of the vibrations, they were able to modify the design and then flew the crap out of the thing.

From 1955 to 1968 the XV-3 was flight tested up to speeds of about 155 knots, successfully completing over 100 conversions from helicopter mode to airplane mode and back. More importantly, the XV-3 program taught engineers and researchers to respect things like aeroelastic whirl flutter and instability. These lessons would prove to be critical in the years to come.

http://www.nationalmuseum.af.mil/factsheets/fac…

http://en.wikipedia.org/wiki/Bell_XV-3

Bell XV-15

In the early 1970s, NASA and the Army teamed up to form the Tilt Rotor Research Aircraft (TRRA) Project Office. The XV-3 program had inspired some pretty smart people at the NASA Ames Research Center to push for a new aircraft to further develop the technology. The major product of the TRRA Project Office would end up being the Bell XV-15.

At first glance, it’s immediately clear that the XV-15 was much more modern than the grand-pappy XV-3. While the earlier machine had a single radial engine in the fuselage, the XV-15 had the now-familiar turbine engines mounted in wing-tip nacelles. They were interconnected through a clever third transmission in the center of the aircraft so that if one engine was lost, power could be transmitted from the remaining engine to prevent asymmetric lift or thrust.

It’s important to note that having the engines rotate about 90 degrees is a tricky thing. You have to take lubrication scavenging at lots of extreme angles into account, as well as fuel and hydraulic flow. There was also a need to have transmission stacks in each nacelle to match the engine output shaft speed with the proprotor speed. For those of you counting, there were 5 transmissions in the aircraft: two in each nacelle and that magic center transmission in the center fuselage.

Other cool features of the XV-15: ejections seats and an on-board Data Acquisition System (DAS). The latter item is really one of the most important parts of the program. You see, the whole point of a flight test program is to gather data. That is your product. That’s what you’re buying with all this hardware and time. The DAS was a computer that collected insane amounts of information from strain gages, accelerometers, thermocouples, and aircraft instruments. It’s an important feature, to say the least. Best of all, it was small enough to fit neatly in the back of a small-ish flying machine.

The first flight of the XV-15 was in 1977 at the Bell Helicopter Flight Test Center in Texas. Bell and Army/NASA test pilots evaluated the aircraft handling qualities and determined it acted exactly as expected. That is to say, it was forgiving, smooth, and easy to fly. This was important since in the 1970s you couldn’t just walk into the local pilot’s lounge and find a guy who could fly a tilt-rotor. There was no POH. No owner’s manual. Just educated guesses about what would happen.

After a planned entry in the 40x80 wind tunnel in 1978, the flight test program kicked into full swing with two aircraft being built (N702NA and N703NA). As the flight test program continued, experiments to really understand the capabilities of tilt-rotor aircraft were performed and a clear operating envelope was developed. In addition, a collection of “best practices” and “lessons learned” was compiled and carefully archived. This information would prove to be priceless towards the end of the 20 th Century when tilt-rotor aircraft really became mature.

I feel it is critical to point out that several mechanical failures occurred during the XV-15 flight test program. In each case, something important was learned, which is the point of flight tests. In 1979 an engine failed during flight in airplane mode, forcing a run-on landing with the nacelles set at 70 degrees. The remaining engine transmitted power through the center transmission to both rotors, as designed, and the landing was noted as uneventful and benign. Another engine failure while hovering in helicopter mode in 1983 resulted in a benign helicopter-style landing. These two unplanned incidents proved that single-engine-out operation (in either airplane or helicopter modes) was no more dangerous than in any other multi-engine aircraft. In addition to these two incidents, numerous smaller issues were safely dealt with including several bird strikes and one tree strike.

There was one other notable incident in 1992 where N702NA rolled over while in a low hover and ended up upside down. The aircraft was destroyed, though the pilots received only minor injuries. The cause for the crash was found to be pure human error. A cotter pin was not installed in the castle nut that secured one of the collective actuators to a control rod. This small oversight cost the life of the 702 airframe. Very sad. I assume that forgetful mechanic was given a stern talking-to.

The XV-15 program ended in 2003 when the 703 airframe was flown from Texas to the Smithsonian. It was landed at the Udzar-Hazy Center, the fuel was drained, and it was parked inside next to the Concorde. It’s still there. Check it out.

https://airandspace.si.edu/collections/ar…

http://www.nasa.gov/centers/dryden…

http://en.wikipedia.org/wiki/XV-15

Bell/Boeing V-22 Osprey

Everyone knows the Osprey. In fact, the V-22 program is well documented (compared to the XV-3 and XV-15) so I’ll keep this brief-ish. You can Google it if you want to know more. I’ll try to keep this post down to the stuff people usually don’t talk about. Or the stuff people SHOULD know and talk about.

As far as I can tell from the rumors floating around here, when the RFP went out for the aircraft that eventually became the V-22, Bell took one look at the XV-15 and said “let’s just scale this little guy up until it fits the spec!” Viola! Of course it was a little more complex than that, but not much. The government had spent plenty of time doing crazy things with the XV-15 like nap-of-the-earth flight and acoustic detection tests. They already liked the machine so they wrote a spec that encouraged aircraft builders to expand on the general concept. This is a fairly common practice in the government and is one of many ways to push the technology envelope without over-constraining it.

When considering the mission, it’s important to remember that the V-22 was designed to do what a CH-46 can do, but be faster and quieter. It can also reach quite a bit further due partly to the inherent efficiency of cruising like an airplane instead of beating the air to death with a helicopter. My favorite part is that they’re eerily quiet when they’re coming at you in airplane mode. Why? A number of reasons. First, when compared to an airplane, the rotors are so big (compared to a propeller) that they can spin very low RPMs. That produces a frequency of noise that humans have a hell of a time detecting. When compared to helicopters, it’s a little more complex. In a chopper, a bunch of the noise you hear is from Blade Vortex Interaction (BVI). This is the WAP WAP WAP noise you hear when one rotor blade hits the trailing tip vortex from the previous blade. Basically, the blades are crashing into their own wakes. That’s noisy. There’s also a bunch of noise coming from the tail rotor too, and it’s a really annoying frequency for humans, but I’ll ignore that for now due to the whole “replace the CH-46” thing.

So what’s all this acoustics talk really mean? The V-22 can get boots on the ground with much less warning than a normal helicopter. That translates directly into fewer American warfighters losing their lives. The “bad guys” will have much less time to prepare a welcoming party for our “peacekeepers” than if they could hear WAP WAP WAP for 5 minutes before the landing.

Also, let’s talk about that speed. A standard helicopter is limited by the laws of physics to about 200 knots in perfect conditions. If you want, I can talk about those physics later. In the real world, they’re much slower. The CH-46 runs out of steam around 150 knots. I have personally spoken with V-22 test pilots that claim the test machines could do in excess of 300 knots. That’s hauling ass for something that can land on the roof of a small building inside a compound far away from the nearest aircraft carrier… Combine that speed with a combat radius of about 400 miles and you can suddenly see how much easier something like the Iran Hostage Situation in 1980 could have been.

Consider this too: if you’re flying along right near the ground in a chopper, you’re a great target for small arms fire and shoulder launched rockets. They heard you coming for several minutes, so they’re going to be ready for you. If you’re going twice as fast, that dick with an AK is going to have to work a lot harder to hit you. Or, unlike a loaded chopper, you can simply fly higher than small arms fire can reach and avoid the problem all together. Boom. Lives saved.

Tilt-rotor Concerns

Most people who are not in any way familiar with the complexities of aeronautical engineering or flight test programs will tell you that the Osprey is bad because CNN says so. Again, on a daily basis I am exposed to the people that develop this technology and use it and I can say with confidence that it is not a bad aircraft. Dig a little into the flight test records of pretty much every major aircraft development program before the Cold War and you’ll read all about test pilots auguring planes into the ground. Keep in mind that a vast majority of aircraft developed in the last 40 years have been a development or refinement of an earlier concept. Very little new aerodynamic ground has been broken, so test pilots and flight test directors have a pretty good idea of what to expect. In short, we have been much more conservative. We stopped pushing the envelope. Granted, the V-22 wasn’t a ground-breaking program (as you can see, we’ve been flying tilt-rotors since the 1950s), but we have only recently started to expose normal men and women to them. We’re seeing the same “mass consumption teething troubles” that ALL airplanes had in the pre-WW2 period. Or that general aviation had in the 1950s. The best thing to do: talk to a V-22 pilot. Go to the next airshow in your area. The Marines love showing them off, so they bring them out pretty often. Strike up a conversation with a V-22 pilot. He’ll be the guy with sunglasses proudly pointing out all the awesome features of his machine. Ask him the hard questions. He won’t mind.

Myth: Vortex ring state (VRS) causes Osprey crashes. The fact is that ALL rotorcraft encounter VRS. Aircraft with higher disk loading (like the Osprey) have a harder time with it than others, but it’s not a surprise. So why did Ospreys crash from VRS back in the day? Two words: Pilot Training. Once pilots were taught that you can’t just ham-fist the controls however you want, the VRS crashes stopped. Some software changes helped too (like traction control keeping grandma out of the ditch in the snow). Helicopter pilots had the same problem back in the 1940s. By the way, VRS is when a helicopter descends into it’s own downwash. When you are producing lift, you are pushing air downwards. This is called downwash and it’s an aerodynamic mess. If you fly a helicopter in it’s own downwash, it’ll have a hell of a time producing lift and it will fall out of the sky like a brick. So will a V-22.

Myth: Since the V-22 can’t autorotate, it will kill everyone. The fact is that it can autorotate a little, but not very well. So what? Neither can a Blackhawk. So it falls like a rock if both engines (bold letters: BOTH) fail. Big deal. How often does even one engine fail? Rarely. Even from bullets. You think autorotation is the miraculous cure for all helicopter engine-out problems? Go find a pilot that has autorotated a UH-1 and also autorotated a UH-60. I’ll wait. I have been blessed enough to have met just such people (more than once) and have heard (more than once) that you can land a Huey with no engine as smooth as if it were powered. The Blackhawk, however, will hit the ground like a ton of bricks. Why? Mostly rotor blade stiffness and stored rotor energy. Blackhawks are meant to be flexible to limit structural loads at the blade root. The blades are also pretty light (~200 lbs each). The Huey was designed to be stiff and have lots of blade tip weight.

The bottom line is that being engineless in a Blackhawk and in a V-22 is just as likely and is just as much of an adventure. You will almost never see a situation where both engines are out in any aircraft that hasn’t already been stuffed in the ground. I’d be more worried about the Blackhawk tail rotor being shot away. Those are pretty fragile. In fact, I’d take the V-22 just because it wasn’t designed in the early 70s. Aircraft crash survival has come a long way, just like automobile crash survival. That’s something else to keep in mind: the Blackhawk, America’s last clean-sheet helicopter design, first flew in 1974. Think it’s time for something a little more modern? I do.

Tilt Rotor Acoustic Model (TRAM)

This little guy is an Easter Egg for you real wind tunnel and acoustic geeks. In 2000 NASA built a ¼ scale V-22 to gather acoustic data for tilt-rotor aircraft. It was a fairly successful test and the lessons learned have been wrapped into the mission strategy used for V-22s in combat. It also helped to qualify the acoustic signature of tilt-rotor aircraft for civil use. You can bet that the engineers at Bell, Boeing, and Augusta-Westland have studied the TRAM test results as they push forward with the AW609 program.

Check out this paper if you’re REALLY feeling like a geek:

http://rotorcraft.arc.nasa.gov/publications/f…

Now you know how much I love these things. I love history, I love machines, and I love the fact that a few (too few) passionate people continue to push for improvements in our technological capabilities. This is America! Let’s push the limits and see what we’re capable of!

deadpedal > Rock Bottom 10/01/2013 at 19:55

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THAT is an awesome write-up. Kudos, sir.

I mostly came here to say, I was a young jarhead in the mid 90s and spent some time helping do some shakedown stuff on the heavy lifting capabilities of the Osprey. I was the correct age to disregard my well-being, but there was always something twitchy about that aspect of its intended purpose.

This was all at Aberdeen and I half expected them to end up in the Chesapeake. Awesome birds, such a feat of "all purpose" thinking, but most of us were a bit unnerved by the things that needed to be tweaked. Glad to have helped and they certainly turned the corner...it was just eye-opening. This is when taken in concert with our experience with lots of helicopters in less than ideal conditions.

And that is anecdotal evidence you can bank on.

Great write-up, thanks for taking the time...

Rock Bottom > deadpedal 10/02/2013 at 10:45

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Thanks a million for your service! The work you did in the 90s will likely help save a few of your fellow Marines down the line. As I've been told, there were a number of "teething" problems with the first batch of Ospreys (software, mostly), which you might have been working with. I hear that they sorted most of that stuff out later in the program. I also feel that the Osprey shouldn't be used for heavy lift stuff. I mean, I guess in case of emergency it should be able to do some heavy lifting, but if it were up to me I would use the big choppers for that kind of stuff and use the Osprey for things like fast deployments or SAR. Thanks again for your service and thanks for the kind words!